The instant invention is the subject matter of Disclosure Document No.: 313364, filed in the PTO on Jul. 20, 1992, and it is respectfully requested that this document be retained beyond the two-year period so that it may be relied upon as evidence of conception of the invention during the prosecution phase of this application, should the need arise.
1. Field of the Invention
This invention relates to amplifier with periodically disabling switch and specially to isolation amplifier.
2. Description of the Prior Art
Generally speaking, capacitors and transformers are usually utilized in alternating current (A.C.) signal coupling. For isolation transfer of differential voltages which include typically, D.C. and low frequency components, the existing arts all apply to modulation-demodulation techniques. At first, the voltage is converted to a high frequency signal through a modulator. This high frequency signal can then be isolatedly transferred by means of capacitor or transformer coupling. After that, the modulated high frequency signal is recovered via a demodulator back to an analog signal that proportionally follows the source voltage. In this manner, the capacitor or transformer plays important roles of both isolation and coupling. It must be noted that some optical devices can play the same role in some applications such as optical coupled isolation amplifiers. In reality, it is also a type of modulation-demodulation system.
Among both of these type reactance elements, the transformer is mostly preferred as a key part of an isolation amplifier, and there are numerous U.S. patents that are examples of such.
In a few applications, instead of transformers, capacitors are utilized, for example, U.S. Pat. Nos. 4,292,595 issued to Lewis R. Smith and 4,748,419 to Thomas A. Somerville. The main advantage and reason for utilizing the capacitor instead of the transformer is that the capacitor performs an integrating process while the transformer can not.
There are some shortcomings that these modulation-demodulation techniques suffer, typically as follows:
1) Nonlinear distortions are often present with these modulation or demodulation processes. Accordingly, this causes degradation of accuracy and dynamic range;
2) The band width of response may be limited at least by carrier frequency; and
3) Circuit design is unduly complex.
A so-called "flying capacitor" technique is in fact a pulse amplitude modulation method. The coupling parts of which consists of a capacitor for data storage and a double pole double throw (DPDT) switch used to generate a data carrier and form a circuit which simultaneously provides isolation between input and output terminals. The DPDT switch alternately connects the capacitor to the inputs for sampling and the outputs for detecting. This kind of arrangement of components is the same as sample-and-hold circuits. The isolation between inputs and outputs is realized by moving contacts of the DPDT switch. Unfortunately, among all kind of DPDT switches, only the mechanical relays can be utilized as high level isolation devices. However, they suffer for some shortcomings such as: the requirement of relative large drive power, low speed, bulkiness and poor reliability.
Accordingly switched-capacitors are devices which perform the function of both storing and transferring charge and are widely applied in high performance differential amplifier and digital converter. Typically, a delta-sigma techniques and an auto-zero techniques have been developed for the promotion of accuracy and stability and improvement of zero shift of amplifiers. A description of the techniques is shown in, for example, the article: "Low-Level MOS Transistor Amplifier using Storage Techniques", R. Poujois, B. Baylac, D. Barbier and J. M. Ittel, Digest of Tech. Papers IEEE Int. Solid-State Circuits Conf., pp. 152-153 Feb. 1973. Yet none of these techniques have used the switched capacitor as a key part to accomplish both coupling and high level isolation while transferring charge.
This invention applies gating storage techniques with capacitors to realize high level isolation sensing of differential voltage and to overcome all shortcomings mentioned above.